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                [Usaco2007 Mar]Ranking the Cows 奶牛排名                </h1>
                <p>时间限制：5s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：64MB</p>			</div>
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                                <h3>题目描述</h3><p><div><span style="font-size: medium">&nbsp;&nbsp;&nbsp;&nbsp;农夫约翰有N(1&le;N&le;1000)头奶牛，每一头奶牛都有一个确定的独一无二的正整数产奶率．约翰想要让这些奶牛按产奶率从高到低排序．&nbsp;&nbsp;&nbsp;&nbsp;约翰已经比较了M(1&le;M&le;10000)对奶牛的产奶率，但他发现，他还需要再做一张关于另外C对奶牛的产奶率比较，才能推断出所有奶牛的产奶率排序．请帮他确定C的最小值．</span></div></p><hr/><h3>输入格式</h3><p><div><span style="font-size: medium">&nbsp;&nbsp;&nbsp;&nbsp;第1行包含两个用空格分开的整数N和M.接下来M行，每行有两个用空格分开的整数X和Y(1&le;X，y&le;1000)，表示奶牛X的产奶率高于奶牛Y.</span></div></p><hr/><h3>输出格式</h3><p><div>&nbsp;</div>
<div><span style="font-size: medium">&nbsp;&nbsp;C的最小值．</span></div></p><hr/><h3>样例输入</h3><pre>5 5
2 1
1 5
2 3
1 4
3 4

INPUT DETAILS:

FJ is comparing 5 cows and has already determined that cow 2 > cow
1, cow 1 > cow 5, cow 2 > cow 3, cow 1 > cow 4, and cow 3 > cow 4
(where the '>' notation means "produces milk more quickly").
</pre><hr/><h3>样例输出</h3><pre>3
</pre><hr/><h3>提示</h3><p><p class="MsoNormal" style="margin: 0cm 0cm 0pt"><span style="font-size: medium"><span lang="EN-US"><font face="Times New Roman">&nbsp;&nbsp;&nbsp;&nbsp;</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">从输入样例中可以发现，约翰已经知道的排名有奶牛</span><span lang="EN-US"><font face="Times New Roman">2&gt;</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">奶牛</span><span lang="EN-US"><font face="Times New Roman">1&gt;</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">奶牛</span><span lang="EN-US"><font face="Times New Roman">5</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">和奶牛</span><span lang="EN-US"><font face="Times New Roman">2&gt;</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">奶牛</span><span lang="EN-US"><font face="Times New Roman">3&gt;</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">奶牛</span><span lang="EN-US"><font face="Times New Roman">4</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，奶牛</span><span lang="EN-US"><font face="Times New Roman">2</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">排名第一．但是他还需要知道奶牛</span><span lang="EN-US"><font face="Times New Roman">1</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的名次是否高于奶牛</span><span lang="EN-US"><font face="Times New Roman">3</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">来确定排名第</span><span lang="EN-US"><font face="Times New Roman">2</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的奶牛，假设奶牛</span><span lang="EN-US"><font face="Times New Roman">1</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的名次高于奶牛</span><span lang="EN-US"><font face="Times New Roman">3</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">．接着，他需要知道奶牛</span><span lang="EN-US"><font face="Times New Roman">4</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">和奶牛</span><span lang="EN-US"><font face="Times New Roman">5</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的名次，假设奶牛</span><span lang="EN-US"><font face="Times New Roman">5</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的名次高于奶牛</span><span lang="EN-US"><font face="Times New Roman">4</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">．在此之后，他还需要知道奶牛</span><span lang="EN-US"><font face="Times New Roman">5</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的名次是否高于奶牛</span><span lang="EN-US"><font face="Times New Roman">3</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">．所以，他至少仍需要知道</span><span lang="EN-US"><font face="Times New Roman">3</font></span><span style="font-family: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">个关于奶牛的排名．</span></span></p></p><hr/><h3>题目来源</h3><p>Gold</p>
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